Artificial bones and methods of making same

ABSTRACT

An artificial bone generally includes an outer wall defining an inner cavity and an inner core disposed within at least a portion of the inner cavity, wherein the inner core includes a porous material having stiffness within a range of stiffness properties for mammalian cancellous bone and strength within a range of strength properties for mammalian cancellous bone.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/917,601, filed May 11, 2007, and U.S. Provisional Application No.60/911,270, filed Apr. 11, 2007, the disclosures of which are herebyexpressly incorporated by reference.

TECHNICAL FIELD

Embodiments of the present disclosure relate generally to artificialbones for use in orthopedic instruction and methods of making the same.

BACKGROUND

In orthopedic surgery, bone and joint cutting or drilling is apreliminary step before the insertion of orthopedic hardware (such aspins or screws) into bones or joints during the repair of a bonefracture or installation of a prosthetic device. Accordingly, orthopedicsurgeons require bone cutting and drilling skills. These skills areobtained by practicing on bones and joints from cadavers, which are veryexpensive and in short supply, or on artificial bones manufactured forsuch practice.

In some instances, surgeons must perform more extensive proceduresbeyond cutting and/or drilling into the bone to set orthopedic hardwarein the bones or joints. For example, after drilling a hole in the boneor joint, the surgeon might administer a bone structure reinforcementcompound, such as a bone cement compound, through a percutaneous orinjection method into the bone or joint. The bone cement penetrates thecancellous bone area. A pin or screw may then be inserted into the hole,and the bone cement hardens therearound to set the pin or screw withinthe reinforced bone. The penetration of the bone cement through anartificial cancellous bone area is limited in current artificial bonesbecause currently designed artificial bones typically include a closedcell artificial cancellous bone area having no interstices or passagesbetween cells for the bone cement to travel through. Moreover, thesecurrently designed artificial bones do not have characteristics andproperties that correspond to other characteristics and properties ofmammalian bones.

Therefore, there exists a need for improved artificial bones and jointsthat perform like mammalian bone when subjected to the proceduresdesigned for fracturing and repairing such bones, for augmenting thebone structure with reinforcing compounds, such as bone cements, and forother static and dynamic biomechanical experimentation.

Moreover, current artificial bones are typically manufactured byreaction injection molding a lower density polyurethane closed cellartificial cancellous bone on a pin or mandrel in a first mold, thenmolding a higher density polyurethane artificial cortical bone aroundthe artificial cancellous bone in a second larger mold, then removingthe pin or mandrel. Because an open cell artificial cancellous bonecannot be reaction injection molded using the same method used for aclosed cell artificial cancellous bone, there also exists a need forimproved methods of making improved artificial bones and joints usingopen cell artificial cancellous bone.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify key features oressential features of the claimed subject matter, not is it intended tobe used to limit the scope of the claimed subject matter.

In accordance with one embodiment of the present disclosure, anartificial bone is provided. The artificial bone generally includes anouter wall defining an inner cavity and an inner core disposed within atleast a portion of the inner cavity, wherein the inner core includes aporous material having stiffness within a range of stiffness propertiesfor mammalian cancellous bone and strength within a range of strengthproperties for mammalian cancellous bone, wherein the inner coreincludes a barrier layer to separate the outer wall from the porousmaterial.

In accordance with another embodiment of the present disclosure, anartificial bone is provided. The artificial bone generally includes anouter wall defining an inner cavity; and an inner core disposed withinat least a portion of the inner cavity, wherein the inner core comprisesa porous material having an apparent modulus of elasticity of at leastabout 26 MPa and ultimate stress of at least about 0.32 MPa.

In accordance with another embodiment of the present disclosure, amethod of making an artificial bone is provided. The method generallyincludes obtaining an inner core, wherein the inner core includes aporous material having stiffness within a range of stiffness propertiesfor mammalian cancellous bone and strength within a range of strengthproperties for mammalian cancellous bone; substantially covering theinner core with a barrier layer; and molding a substantially continuousouter wall around the inner core.

In accordance with another embodiment of the present disclosure, amethod of making an artificial bone is provided. The method generallyincludes obtaining first and second outer wall portions, wherein each ofthe first and second outer wall portions define a portion of an innercavity such that when the first and second outer wall portions areassembled into an outer wall, the outer wall defines an inner cavity;obtaining a hardened inner core configured to fit within at least aportion of the inner cavity; inserting the inner core within at least aportion of the inner cavity of the first outer wall portion; andattaching the second outer wall portion to the first outer wall portionto form the outer wall having the inner core disposed within the atleast a portion of the inner cavity.

In accordance with another embodiment of the present disclosure, amethod of making an artificial bone is provided. The method generallyincludes obtaining an outer wall portion, wherein the outer wall portiondefines at least a portion of an inner cavity; obtaining an inner coreconfigured to fit within at least a portion of the inner cavity, whereinthe inner core comprises a formable porous material; inserting the innercore within at least a portion of the inner cavity of the first outerwall portion; and hardening the inner core after inserting the innercore within the at least a portion of the inner cavity of the outer wallportion.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisdisclosure will be more readily understood by reference to the followingdetailed description, when taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a partial cut-away view of an artificial bone formed accordingto various aspects of the present disclosure;

FIG. 2 is an exploded view of the artificial bone of FIG. 1;

FIG. 3 is a cross-sectional view of the artificial bone taken throughthe plane 3-3 of FIG. 1;

FIG. 4 is a partial cut-away view of an artificial bone of FIG. 1 formedaccording to other aspects of the present disclosure;

FIGS. 5-9 show a method according to various aspects of the presentdisclosure of making an artificial bone having an open cell artificialcancellous bone and a substantially continuous artificial cortical bone;

FIG. 10A is micro computed tomography (micro-CT) rendering of humanvertebral cancellous bone;

FIGS. 10B and 10C are micro computed tomography (micro-CT) renderings ofartificial cancellous bone formed according to various aspects of thepresent disclosure;

FIG. 11 is a modulus degradation curve, plotting apparent modulus versuslife fraction of human cancellous bone and artificial cancellous boneformed according to various aspects of the present disclosure;

FIG. 12 is a S-N stress curve, plotting effective strain (S) versusnumber of cycles to failure (N) of mammalian cancellous bone,closed-cell foam, and artificial cancellous bone formed according tovarious aspects of the present disclosure; and

FIGS. 13-15 shows a prior art method of making an artificial bone havingclosed cell artificial cancellous bone.

DETAILED DESCRIPTION

Embodiments of the present disclosure are generally directed toartificial bones and methods of making these bones. An artificial bone20 constructed in accordance with one embodiment of the presentdisclosure may be best understood by referring to FIG. 1. The bone 20includes an outer wall 22 having an inner cavity 24, and an inner core26 disposed within at least a portion of the inner cavity 24. Asdescribed in greater detail below, the outer wall 22 and the innercavity 24 are designed and configured to have characteristics andproperties that are similar to mammalian bone when subjected to theprocedures designed for fracturing and repairing bones, including bonesand bone joints, and when subjected to dynamic biomechanicalexperimentation.

The artificial bone 20 is a manufactured alternative to mammaliancadaver bones for use in orthopedic instruction and experimentation.Accordingly, the artificial bone 20 is designed to simulate mammalianbone when broken and subjected to repair, for example, when subjected tocutting, drilling, and/or the injection of bone structure reinforcingcompounds, such as bone cement compounds. In addition, the artificialbone 20 offers an alternative to human bones for examining theeffectiveness of cementitious compounds to be used in bone repairprocedures or in cement augmentation for osteoporotic bones. Further,the artificial bone 20 offers an alternative to human bones for staticand dynamic biomechanical experiments. For example, artificial bones canbe used to test the life cycle and durability of prostheses, artificialjoints, orthopedic hardware and devices, etc. Under the conditionsdescribed above, artificial bones are a preferable alternative tocadaver bones because they have reduced inter-specimen variation, do notdegrade over time, and are generally lower in cost in comparison tocadaver bones.

In the illustrated embodiment of FIGS. 1-3, there is shown an artificialhuman humerus bone 20; however, it should be understood that allmammalian bones and joints, in addition to the human humerus bone, arewithin the scope of the present disclosure. The properties of the outerwall 22, inner cavity 24, and inner core 26 of the artificial bone 20are all designed to have characteristics and features similar tomammalian cortical bone, medullar cavity, and cancellous bone, as willnow be described in greater detail. Depending on the type of bone, suchcharacteristics and features may include standard morphologicalproperties of mammalian bone, such as volume fraction, surface to volumeratio, trabecular number, trabecular thickness, trabecular spacing orcell size, intercept length, connectivity index, degree of anisotropy,and characterization of the bone. In addition, such characteristics andfeatures may include compressive properties of mammalian bone, such asapparent density, stiffness, measured as apparent modulus of elasticity,and strength, measured as ultimate stress.

The outer wall 22 is designed and configured to have characteristics andfeatures similar to mammalian cortical bone when subjected to theprocedures designed for fracturing and repairing bones and joints andwhen subjected to biomechanical experimentation. In that regard, theouter wall 22 may be made from a rigid, fracturable and drillablematerial, such as a rigid polymer material. Suitable materials include,but are not limited to, thermoplastics and thermosets, such aspolyurethanes, resins, fiberglass, fiberglass filled resins and epoxies,and other suitable materials. In one embodiment, the outer wall 22 isreaction injection molded from a liquid polyurethane foam that istransferred to a mold for the outer wall. The thickness of the outerwall 22 may also be designed and configured to simulate mammalian bone.In that regard, a thicker outer wall 22 is generally used to simulatehealthy mammalian bones, while a thinner outer wall 22 is generally usedto simulate osteoporotic mammalian bones. The thickness of an artificialosteoporotic bone is generally about 50% to about 80% less than thethickness of an artificial healthy bone.

The inner cavity 24 is designed and configured to have characteristicsand features similar to a mammalian medullar cavity when subjected tothe procedures designed for fracturing and repairing bones and whensubjected to biomechanical experimentation. In the illustratedembodiment, the inner cavity 24 is shown as an empty cavity. However, itshould be understood that the inner cavity may include a spacer orbladder and/or may be filled with suitable materials for ease ofmanufacturing or for other design or functional factors.

As shown in FIGS. 1-3, the inner core 26 is disposed within at least aportion of the inner cavity 24. The inner core 26 is also designed andconfigured to have similar characteristics and features as mammaliancancellous bone (or trabecular bone) when subjected to the proceduresdesigned for fracturing and repairing bones and when subjected tobiomechanical experimentation. In that regard, the inner core 26generally has a porous, open cell structure, including interstices orpassages between cells. These interstices are designed to be permeableto high-viscosity fluids, such as air, water, and other fluids. Beingpermeable to fluids, the interstices are designed to be penetrated by abone structure reinforcing compound, such as a bone cement compound,when the compound is injected or otherwise introduced into the region,for example, for the purposes of cement augmentation or settingorthopedic hardware and devices (including, but not limited to, pins,screws, wires, rods, anchors, prostheses, artificial joints, jointrepair hardware, and other devices used to repair bones).

As seen in the illustrated embodiment of FIG. 9, the inner core 326 mayinclude a barrier layer 342 to prevent outer wall 322 molding materialsfrom penetrating the porous, open cell structure of the inner core 326during manufacture, as described in greater detail below. The barrierlayer 342 may be an plastic film layer that is impermeable to themolding materials of the outer wall 322 used to cover the outer surfaceof the inner core 326. As a non-limiting example, the barrier layer 324may be a urethane film.

An inner core 26 formed in accordance with embodiments of the presentdisclosure has morphological properties that are similar to or within arange of properties for mammalian cancellous bone, such as volumefraction, surface to volume ratio, connectivity index, andcharacterization, as described in greater detail below in EXAMPLE 1. Inaddition, an inner core 26 formed in accordance with embodiments of thepresent disclosure has stiffness, strength, and density properties thatare within the range of properties for mammalian cancellous bone whentested under compression. The range of compressive properties formammalian cancellous bone were compiled from mean data for 1133compressive test samples performed on either cylindrical cores or cubespecimens of human cancellous bone taken from vertebrae, femurs, andtibias from cadaver bone aged 20-100 years, using a method with uniaxialcompression loading at various strain rates. Large ranges and variationsin the mechanical properties of human cancellous bone, as shown in TABLE2 in EXAMPLE 2 below, may be a result of differences in subject age,degeneration, bone density, and source of bone (e.g., vertebral, tibial,etc.).

The human cancellous bone data was compiled from the followingdocuments, the disclosures of which are hereby incorporated byreference:

-   Gibson, L. J., “The Mechanical Behavior of Cancellous Bone,” J.    Biomechanics 18(5):317-328, 1985;-   Goldstein, S. A., et al. “The Mechanical Properties of Human Tibial    Trabecular Bone As a Function of Metaphyseal Location,” J.    Biomechanics 16(12):965-969, 1983;-   Keller, T. S., “Predicting the Compressive Mechanical Behaviour of    Bone;” J. Biomechanics 27(9):1159-1168, 1994;-   Kopperdahl, D. L., Keaveny, T. M., “Yield Strain Behavior of    Trabecular Bone,” J. Biomechanics 31:601-608, 1998; and-   Kopperdahl, D. L., et al. “Quantitative Computed Tomography    Estimates of the Mechanical Properties of Human Vertebral Trabecular    Bone,” J. Orthop. Res. 20:801-805, 2002.

Additional sources of human cancellous bone data include the followingdocuments, the disclosures of which are hereby incorporated byreference:

-   Morgan, E. F., Bayraktar, H. H., Keaveny, T. M., “Trabecular bone    modulus-density relationships depend on anatomic site,” J. Biomech.    36:897-904, 2003; and-   Hernandez, C. J., Keaveny, T. M., “A biomechanical perspective on    bone quality,” J. Biomech. 39:1173-1181, 2006.

While it should be appreciated that there are large ranges andvariations in the mechanical properties of human cancellous bone, theinventors have found that certain ranges of stiffness, strength, andapparent density properties have characteristics and features similar toa mammalian medullar cavity when subjected to the procedures designedfor fracturing and repairing bones and when subjected to biomechanicalexperimentation. In that regard, suitable ranges for these propertiesare shown in TABLE 2 in EXAMPLE 2 and described in greater detail below.The inventors have further found that stiffness and strength propertiesincrease with increased apparent density. The inventors have furtherfound that the upper limit for apparent density is defined by theopenness of the interstices and passages between cells of the open cellstructure required to allow for permeation of a bone structurereinforcing compound, such as a bone cement compound. If the apparentdensity of the open cell structure becomes too high, the cells are notopen enough to allow penetration of a bone cement compound.

Regarding stiffness, in one embodiment of the present disclosure, theinner core 26 has an apparent modulus of elasticity of at least about 26MPa. In another embodiment, the inner core 26 has an apparent modulus ofelasticity in a range of about 26 MPa to about 673 MPa. In anotherembodiment, the inner core 26 has an apparent modulus of elasticity in arange of about 55 MPa to about 535 MPa. In another embodiment, the innercore 26 has an apparent modulus of elasticity in a range of about 26 MPato about 200 MPa. In yet another embodiment, the inner core 26 has anapparent modulus of elasticity in a range of about 26 MPa to about 100MPa.

Regarding strength, in one embodiment of the present disclosure, theinner core 26 has strength, measured as ultimate stress, of at leastabout 0.32 MPa. In another embodiment, the inner core 26 has ultimatestress in a range of about 0.32 MPa to about 46 MPa. In anotherembodiment, the inner core 26 has ultimate stress in a range of about0.34 MPa to about 12.08 MPa. In another embodiment, the inner core 26has ultimate stress in a range of about 0.32 MPa to about 1.0 MPa. Inyet another embodiment, the inner core 26 has ultimate stress in a rangeof about 0.32 MPa to about 0.60 MPa.

Regarding apparent density, in one embodiment of the present disclosure,the inner core 26 has an apparent density of at least about 0.09 g/cm³.In another embodiment, the inner core 26 has an apparent density in arange of about 0.09 g/cm³ to about 0.64 g/cm³. In another embodiment,the inner core 26 has an apparent density in a range of about 0.09 g/cm³to about 0.49 g/cm³. In another embodiment, the inner core 26 has anapparent density in a range of about 0.12 g/cm³ to about 0.49 g/cm³. Inyet another embodiment, the inner core 26 has an apparent density in arange of about 0.15 g/cm³ to about 0.49 g/cm³.

It should be appreciated that osteoporotic cancellous bone has lowerstiffness and/or strength properties when tested under compressioncompared to healthier, non-osteoporotic bones. In that regard,osteoporotic cancellous bone generally has an apparent modulus ofelasticity values of less than about 100 MPa and ultimate stress valuesof less than about 2 MPa, while healthier, non-osteoporotic cancellousbone generally has an apparent modulus of elasticity values of greaterthan about 100 MPa and ultimate stress values of greater than about 2MPa.

Likewise, osteoporotic cancellous bone generally has a lower apparentdensity compared to healthier, non-osteoporotic cancellous bone. In thatregard, osteoporotic cancellous bone generally has an apparent densityof less than about 0.20 g/cm³, while healthier, non-osteoporoticcancellous bone generally has an apparent density of greater than about0.20 g/cm³. In accordance with these properties, inner cores formed inaccordance with the foregoing embodiments (including inner cores thatsimulate both osteoporotic and healthy mammalian cancellous bone) mayshow some crumbling when subjected to fracturing, cutting, drilling, orthe insertion of hardware, such as pins and/or screws, similar to theproperties of mammalian cancellous bone subjected to the sameprocedures.

In one embodiment of the present disclosure, an inner core 26 that hasproperties and characteristics corresponding to mammalian cancellousbone, as described above, is a hardened reticulated open cell foam. Inthat regard, the inner core 26 is a dry, formable, reticulated open cellfoam, such as an open cell polyurethane, polyester, or other suitableopen cell foam, for example, polyurethane open cell foam having about 14ppi (pores per inch) and being about 95% open, manufactured by the E.N.MURRAY CO. and sold as product PTA 14 ppi Natural, having a density ofabout 0.023 g/cm³. In other embodiments, the open cell foam has poresper inch in the range of about 10 to about 45 pores per inch.

The reticulated open cell foam begins as a formable or pliable foam, butis impregnated with an inner core hardening agent, such as resin,ceramic, or metal, including alloys or oxides thereof, to fabricatesuitable strength, stiffness, and density properties for the foam tosimulate mammalian cancellous bone, as described in greater detailabove. Such impregnation of the foam may be accomplished by dipping,saturating, coating, or injecting the inner core hardening agent, or byany other method of impregnation by a fluid hardening agent. As theimpregnated hardening agent cures within the foam open cell structure,the once-formable foam becomes a hardened foam having strength,stiffness, and density properties in accordance with the presentdisclosure. A suitable amount of hardening agent applied to the opencell foam achieves stiffness, strength, and apparent density propertiesthat are similar to or within a range of properties for mammaliancancellous bone, while also maintaining a suitable open cell structureto allow a bone structure reinforcing compound, such as bone cement, topenetrate through the open cells.

In a preferred embodiment, the formable open cell foam is impregnatedwith epoxy, urethane, silicon, ceramic, or other suitable hardeningresins. As nonlimiting examples, suitable epoxy resins include, but arenot limited to, marine grade epoxy resin, manufactured by TAP®,including resin #314 blended with B-side hardener #109, RENLAM®4017resin blended with Ren®1510 hardener manufactured by HUNTSMAN™, andepoxy resin systems manufactured by COTRONICS CORPORATION.

The resin can be suitably combined with filler materials to increase thestrength properties of the hardened open cell foam. Suitable fillermaterials include, but are not limited to, glass fiber, such as 3032milled e-glass fiber, manufactured by FIBERTEC™, carbon fiber, micro- ornano-sized fillers, and nanocrystalline metals and alloys, such as thosemanufactured by POWERMETAL TECHNOLOGIES, INC. As a non-limiting example,a suitable metal-filled resin is EC-433 High Temp Epoxy Casting SystemAluminum Filled and EC-433-2 Hardener, manufactured by ADTECH PLASTICSYSTEMS. As an alternative to resin hardening, the open cell foam may behardened with castable ceramics, such as those manufactured under thebrand name RESCOR™, including products 750 and 780.

As described in EXAMPLE 2 below, the inventors discovered that an opencell foam impregnated with an aluminum-filled epoxy resin with anapparent density of 0.31 g/cm³ resulted in about 6-fold improvements inapparent modulus and ultimate stress values compared to the same opencell foam impregnated with an aluminum-filled epoxy resin with anapparent density of 0.15 g/cm³ (see TABLE 2, RD3 and RD4 data). Theinventors further discovered that such impregnation with analuminum-filled epoxy resin having an apparent density of 0.31 g/cm³still achieves openness in the interstices and passages between cells toallow for permeation of a bone structure reinforcing compound, such as abone cement compound.

In one embodiment of the present disclosure, a method for impregnatingand hardening a dry, formable open cell foam with an inner corehardening agent includes the following method steps. A liquid hardeningagent mixture is prepared, such as a resin, ceramic, or metal, includingalloys or oxides thereof. The weight of the dry, formable open cell foamis then recorded. The dry, formable open cell foam sample is thensaturated with the liquid hardening agent. Excess liquid hardening agentis then removed from the wet, saturated, formable foam sample, forexample, by being squeezed through a roller system that acts like asqueegee to remove excess hardening agent. While it is important thatthe open cell foam be saturated with liquid hardening agent, an excessof liquid hardening agent may affect the “openness” of the cells. Theweight of wet, saturated, formable open-cell foam sample is thenrecorded to determine whether a proper amount of saturation hasoccurred. For example, if the weight of the wet, saturated, formableopen-cell foam is too high or low, then the weight can be adjusted bychanging the distance between the roller system and repeating thesqueegee process until the wet sample of foam is within a specifiedweight range based on dry density, such that dry density is within therange of density values for mammalian cancellous bone. The wet,saturated, formable open cell foam is then dried in a ventilated areauntil the hardening agent has cured. It should be appreciated that themethod steps described herein for impregnating and hardening a dry,formable open cell foam with a hardening agent are exemplary in natureand not intended to be limiting.

As discussed above, a suitable hardened reticulated open cell foam, asdescribed above, can be used as the inner core 26 of an artificial bone20 in accordance with embodiments of the present disclosure. When inuse, for example, for orthopedic repair and/or augmentation, a user candrill or cut into the bone 20 using standard orthopedic tools andcutting and drilling devices. After forming a hole in the bone 20, abone structure reinforcing compound, such as a suitable cementitiouscompound used for orthopedic repair and augmentation as described above,can be injected into the inner cavity 24 of the bone 20. The bone cementpermeates the inner cavity 24, and because the inner core 26 is ahardened, porous material having open passages throughout its structure,the bone cement is able to permeate the portion of the inner core 26adjacent the hole. As a non-limiting example, orthopedic hardware (suchas an orthopedic screw) can then be inserted into the hole, and the bonecement will harden around the hardware to set the hardware within thecement-reinforced inner core 26. As another non-limiting example, thebone cement can be used to augment the cancellous bone structure.

In accordance with embodiments of the present disclosure, methods ofmanufacturing artificial bones will now be described. Referring to FIGS.13-15, the manufacture of artificial bones 420 having closed cellartificial cancellous bone generally includes molding, for example, byreaction injection molding, an inner core 426 of artificial cancellousbone using an inner core mold (not shown). A suitable spacer 440, shownas a pin, extending longitudinally through the inner core 426, may beused to define an inner cavity 424 (FIG. 15) in or adjacent the innercore 426. The inner core 426 may be reaction injection molded from a lowdensity polyurethane foam material, for example, having a density in therange of about 0.08 to about 0.24 g/cm³.

After the inner core 426 has been molded around the pin 440, the innercore 426 and the pin 440 are placed in a second larger mold (not shown),and an outer wall 422 is injection molded around the inner core 426 andthe pin 440 (see FIG. 14). The outer wall 422 may be molded from ahigher density polyurethane material than the inner core 426, forexample, having a density in the range of about 0.32 to about 0.48g/cm³. After the outer wall 422 has been molded, the artificial bone 420is removed from the mold and the pin 440 is removed from the artificialbone 420 (see FIG. 15).

If this method was used with an inner core comprising an open cellartificial cancellous bone, the polyurethane of the outer layer wouldpermeate the interstices of the open cell artificial cancellous bone,rendering it filled and useless as an open cell structure. Because anopen cell artificial cancellous bone cannot be reaction injection moldedusing the method described above for a closed cell bone 420, thefollowing methods are directed to improved methods of making improvedartificial bones and joints using open cell artificial cancellous bone.

A first method of manufacturing artificial bones will be described withreference to FIG. 2. The method includes forming first and secondportions 30 and 32 of the outer wall 22. It should be understood thatsuch first and second portions 30 and 32 may be formed as separateportions or as an integral outer wall 22, which is later cut into firstand second portions 30 and 32. The outer wall 22, including any separateportions, may be formed by reaction injection molding or any othersuitable forming method. If formed by reaction injection molding, itshould be appreciated that a suitable spacer (not shown) may be insertedin the mold to define an outer wall 22 having an inner cavity 24 (seeFIG. 1) during the molding process.

In the illustrated embodiment of FIG. 2, the first and second outer wallportions 30 and 32 are divided generally along the longitudinal axis ofthe bone 20. It should be understood, however, that the first and secondouter wall portions 30 and 32 need not be divided along the longitudinalaxis of the bone 20, but can be divided along any sectional portionsthat provide access to the inner cavity 24 of the bone 20. It shouldfurther be understood that a bone having more than first and secondouter wall portions is also within the scope of this disclosure. Each ofthe first and second outer wall portions 30 and 32 define a portion ofan inner cavity 24 such that, when the first and second outer wallportions 30 and 32 are assembled into a complete outer wall 22, theouter wall 22 defines an inner cavity 24 (see FIG. 1).

The method further includes forming an inner core 26 configured to fitwithin the inner cavity 24 (see FIG. 1), wherein the inner core 26 hashardness and porosity properties that are similar to mammalian bone, asdescribed above. In accordance with the first method, the inner core 26is hardened, for example, by resin impregnation and curing, before beingformed to have characteristics and features similar to cortical bone,such that the inner core suitably fits within at least a portion of theinner cavity 24 of the bone 20. In accordance with this method, thehardened inner core 26 can be formed by being machined, cut, ground,thermoformed, molded, or otherwise formed in accordance with any othersuitable forming methods.

In addition, the method includes inserting the inner core 26 within aportion of the inner cavity 24 of the first outer wall portion 30 andattaching the second outer wall portion 32 to the first outer wallportion 30 to form the outer wall 22 having an inner core 26 disposedwithin at least a portion of the inner cavity 24. The attachment of thefirst and second outer wall portions 30 and 32 may be by adhesive, heatsealing, mechanical attachment means, or any other suitable attachmentmechanism. Oftentimes, a mold release formula is applied to release thefirst and second outer wall portions 30 and 32 from their respectivemolds. The inventors have found that such mold release formula may needto be removed from the first and second outer wall portions 30 and 32before using adhesive for attachment.

A second method of manufacturing artificial bones will be described,also with reference to FIG. 2. The second method is substantiallysimilar to the first method, except that the inner core 26 is a formableor pliable, porous material, and additional forming and hardening of theinner core 26 takes place after the inner core 26 has been fit or placedwithin the inner cavity 24 of the bone 20. In accordance with the secondmethod, the porous, formable inner core 26 is formed and placed withinat least a portion of the inner cavity 24. The inner core 26 issubsequently hardened (for example, by resin impregnation and curing)after being placed within the inner cavity 24. Because the inner core 26is a porous material, as the inner core hardening agent (such as resin)is introduced into the inner cavity 24, the hardening agent readilypermeates throughout the inner core 26 within the inner cavity 24. Asthe hardening agent cures, the inner core 26 is hardened to specificparameters in accordance with the present disclosure.

In accordance with the second method, it should be appreciated that theinner core 26 need not be formed to fit exactly within the inner cavity24 prior to placement in the inner cavity 24, but rather is additionallyformed as a result of its conforming to the shape of the inner cavity 24into which it is placed, then hardened to maintain its form. In analternative embodiment, the inner core 26 may be saturated with an innercore hardening agent, such as resin, outside the inner cavity 24, theninserted into the inner cavity 24 before the hardening agent has cured.

It should be appreciated that in accordance with this method, theporous, formable inner core 26 may be placed between the first andsecond outer wall portions 30 and 32, which are then attached to oneanother to form an artificial bone 20. In an alternate embodiment, asseen in FIG. 4, the inner core 126 may be inserted (or squeezed) intothe inner cavity 124 through an opening 134 in the outer wall 122, i.e.,instead of dividing the outer wall 122 into one or more portions, asdescribed above. It should further be appreciated that, when insertedthrough an opening 134 in the outer wall 122, the inner core 126 may besaturated with an inner core hardening agent outside the inner cavity124, then inserted into the inner cavity 124 before the hardening agenthas cured, or the inner core hardening agent may be applied to the innercore 126 in the inner cavity 124 through the same opening 134. After theinserting and hardening steps, the opening 134 in the outer wall 122 canbe patched with suitable patching materials.

Further in accordance with embodiments of the present disclosure, athird method of manufacturing artificial bones will now be describedwith reference to FIGS. 5-9. The third method is substantially similarto the first method, except that the outer wall 322 (see FIG. 9) isformed as a substantially continuous outer wall around the inner core326 (see FIG. 5). A suitable substantially continuous outer wall 322 issubstantially free of seams, for example, along the longitudinal axis ofthe bone 20.

Referring to FIG. 5, an inner core 326 is designed configured to fitwithin an inner cavity 324 of an outer wall 322 (see FIG. 9). The innercore 326 has hardness and porosity properties that are similar tomammalian bone, as described above. The inner core 326 may include asuitable spacer 344 to maintain an empty space for the inner cavity 324.In the illustrated embodiment, the spacer 344 is shown as a hollowcylinder, which may be suitably made from similar materials as the outerwall 322. It should be appreciated, however, that suitable spacers mayinclude air-filled balloons or bladders positioned adjacent the innercore 326.

As seen in FIG. 6, the inner core 326 may include a barrier layer 342.In that regard, a barrier layer 342 is formed over the inner core 326and the spacer 344. As a non-limiting example, a suitable barrier layeris a plastic film layer, such as a thin urethane film layer. In theillustrated embodiment of FIG. 6, the inner core 326 and spacer 344 aresealed between first and second plies of a suitable barrier layer 342.The barrier layer 342 can then be heat pressed or vacuum sealed to theinner core 26 and spacer 344 to form a barrier layer around the innercore 26. It should be appreciated that an optional adhesive may be usedon the first and second plies of the barrier layer 342 to promoteadhesion between the barrier layer 342 and the inner core 326. As seenin FIG. 7, excess barrier layer material can be cut away from thejoining seam 346 between the first and second plies of the barrier layer342.

In accordance with this method, first and second outer wall portions 30and 32, as shown in FIG. 2, may be used as spacers to aid in the processof forming the barrier layer 342 around the inner core 326. In thatregard, first and second outer wall portions 30 and 32 are placed in amold, first and second plies of the barrier layer 342 are draped overthe outer wall portions 30 and 32, and the inner core 326 is placed inone of the first and second outer wall portions 30 and 32. The barrierlayer 342 can then be heat pressed or vacuum sealed within first andsecond outer wall portions 30 and 32 to form a barrier layer around theinner core 26. After the barrier layer 342 has been applied to the innercore 26, the mold can be reopened, and the first and second outer wallportions 30 and 32 can be removed from the mold.

Referring now to FIGS. 8 and 9, a substantially continuous outer wall322, i.e., an outer wall substantially free of seams, is formed aroundthe inner core 326 and barrier layer 342. The outer wall 322 may beconfigured to cover the joining seam 346 between the first and secondplies of the barrier layer 342 to ensure outer wall strength along thebarrier layer seam 346. In that regard, the inner core 26 having abarrier layer 342 may be returned to the mold used to form the barrierlayer 342. In the absence of the first and second outer wall portions 30and 32, which were used as spacers in the mold, the mold can be filledwith an outer layer molding material, such as liquid urethane orfiberglass filled epoxy, around the inner core 326 and barrier layer342. The molding material cures around the inner core 26 and forms asubstantially continuous outer wall 22 around the inner core 26, i.e.,an outer wall substantially free of seams.

Artificial bones made with a substantially continuous outer wall aresignificantly advantageous over artificial bones made by securing firstand second outer wall portions 30 and 32 to one another, for example,with adhesive applied generally along the longitudinal axis of the bone20. In that regard, artificial bones having a substantially continuousouter wall have a reduced failure rate, for example, when subjected todrilling or cutting in orthopedic practice and biomechanical testing, asa result of the absence of a seam along the longitudinal axis of thebone. Moreover, this method eliminates the step of removing the moldrelease formula which tends to prevent strong adhesive attachmentbetween first and second outer wall portions 30 and 32.

A number of tests were performed under properly controlled conditions toinvestigate the characteristics and properties of the artificial bonesformed in accordance with embodiments described herein. These testsresults, as well as comparative data for human cancellous bone collectedfrom several published sources, are included below in EXAMPLES 1-4. Inthat regard, EXAMPLE 1 includes standard morphological data forartificial cancellous bone formed in accordance with embodiments of thepresent disclosure and similar data for human cancellous bone obtainedfrom human vertebrae. EXAMPLE 2 includes static compressive data forsamples of artificial cancellous bone formed in accordance withembodiments of the present disclosure and similar data for humancancellous bone. EXAMPLE 3 describes results of fatigue tests performedon artificial cancellous bone formed in accordance with embodiments ofthe present disclosure and similar data for human cancellous bone.EXAMPLE 4 describes results from the effective strain tests performed onartificial cancellous bone formed in accordance with embodiments of thepresent disclosure and similar data for bovine trabecular bone, humanvertebral bone, and closed cell foam.

EXAMPLE 1 Standard Morphological Parameters

Standard morphological parameters of exemplary hardened reticulated opencell foam formed in accordance with embodiments of the presentdisclosure and data for human cancellous bone obtained from humanvertebrae are shown below in TABLE 1. In TABLE 1, the followingabbreviations are used: “BV/TV” is volume fraction; “BS/TV” is surfaceto volume ratio (in mm⁻¹); “Tb.N” is trabecular number (in mm⁻¹);“Tb.Th” is trabecular thickness (in mm); “Tb.Sp” is trabecular spacingor cell size (in mm); “MIL1,2,3” is mean intercept length (in mm); “TCI”is connectivity index (in mm⁻¹); “DA” is degree of anisotropy and isequal to the maximum mean intercept length (i.e., one of MIL1, MIL2, andMIL3) divided by the minimum mean intercept length (i.e., one of MIL1,MIL2, and MIL3); and a morphology characterization of “TI” is transverseisotropic.

In the following examples, samples #1521-55 and #1521-59 were bothprepared using the same process, including the following method steps:(1) preparing an epoxy mixture (specific mixture described below); (2)recording the weight of the dry formable open cell foam having 14 ppi(pores per inch), manufactured by the EN MURRAY COMPANY, and sold asproduct PTA 14 ppi Natural having a density of 0.023 g/cm³; (3) soakingthe formable open cell foam sample with the epoxy mixture; (4) feedingthe saturated foam sample through two aluminum rollers that act like asqueegee to remove excess epoxy; (5) recording the weight of wetformable open-cell foam sample; (6) if the weight of the wet formableopen-cell foam was too high or low, then adjusting the distance betweenthe rollers and repeating the squeegee process until the sample of foamis within a specified weight range, based on dry density; and (7)hanging the open cell foam samples in a ventilated area until the wetfoam has cured.

Sample #1521-55 includes marine grade epoxy resin, manufactured by TAP®,including resin #314 blended with B-side hardener #109. Sample #1521-59includes marine grade epoxy resin, manufactured by TAP®, including resin#314 blended with B-side hardener #109, combined with 3032 millede-glass fiber, manufactured by FIBERTEC™, in a 1:1 ratio with the resinportion of the epoxy system.

The data on morphological parameters for human vertebral cancellous bonesample sections, included in TABLE 1 below, were obtained from thefollowing three sources: O. Cviganovic et al., “Age- andRegion-Dependent Changes in Human Lumbar Vertebral Bone,” SPINE29(21):2370-2375, 2004 (“Data 1”); a University of Vermont, Departmentof Mechanical Engineering study on micro-CT trabecular bone morphology,conducted by T. S. Keller (2004) (“Data 2”); and Parkinson et al.,“Interrelationships Between Structural Parameters of Cancellous BoneReveal Accelerated Structural Change at Low Bone Volume,” J. Bone &Mineral Res., Vol. 18 (2003) (“Data 3”), the disclosures of which arehereby incorporated by reference. The data in the column labeled “Data1” were obtained from 48 samples of human cadaveric L3 vertebralcancellous bone; the data in the column labeled “Data 2” were obtainedfrom 8 samples of human cadaveric L1-L5 vertebral cancellous bone; andthe data in column of labeled “Data 3” were obtained from 280 samples ofhuman cadaveric cancellous bone from all skeletal sites.

TABLE 1 STANDARD MORPHOLOGICAL PARAMETERS Human Cancellous Bone Data 1Data 2 Data 3 Rigid Open Cell Foam (Thoracic, (n = 8, Lumbar, (n = 280,all #1521-55 #1521-59 Vertebral) Vertebral) skeletal sites) BV/TV 0.0790.106 0.095 0.07-0.13 0.036-0.344 BS/BV (mm⁻¹) 6.263 5.298 13.6710.51-13.24 8.299-29.85 Tb · N (mm⁻¹) 0.249 0.280 0.985 0.48-0.740.254-1.03  Tb · Th (mm) 0.319 0.378 0.146 0.15-0.19 0.067-0.241 Tb · Sp(mm) 3.70 3.20 1.000 1.16-1.91 0.321-2.099 MIL1 (mm) 0.787 0.924 0.9960.38-0.50 — MIL2 (mm) 0.773 0.856 1.399 0.36-0.45 — MIL3 (mm) 0.7050.808 1.018 0.30-0.41 — TCI (mm⁻¹) 0.258 0.157 — 0.08-0.26 — SMI (mm⁻¹)1.971 DA 1.12 1.14 1.37 1.08-1.40 Morphology TI TI TI TI —

The results shown above in TABLE 1 are based on micro computedtomography (micro-CT) data of hardened reticulated open cell foamsamples #1521-55 and #1521-59. The data shows that the hardenedreticulated open cell foam formed in accordance with embodiments of thepresent disclosure has some similar morphology properties to the datacollected for human vertebral cancellous bone. For example, volumefraction (BV/TV), surface to volume ratio (BS/TV), and connectivityindex (TCI) values for samples #1521-55 and #1521-59 are all within theranges for the human vertebral cancellous bone Data 2.

In addition, volume fraction (BV/TV) values for samples #1521-55 and#1521-59 are both within the range for the human cancellous bone Data 3,and volume fraction (BV/TV) values for sample #1521-59 are within theranges for human vertebral cancellous bone Data 1 and Data 2.

In addition, the appearance of the rigid open-cell foam samples #1521-55and #5121-59 and human cancellous bone are similar. In that regard, thecharacterization of the bone as transverse isotropic (TI) is the same insamples #1521-55 and #1521-59 and that of the vertebral bone dataprovided in Data 1 and Data 2. Moreover, both artificial and human bonehave interconnected networks of rods or trabeculae, as best seen inFIGS. 10A-10C. Referring to FIG. 10A, the structure of the rigid opencell foam comprises an integrated network of thin, interconnectedartificial bone trabeculae. However, when compared to the trabeculae inhuman cancellous bone (see FIGS. 10B and 10C), the artificial trabeculaehave thicker and more widely spaced elements. In that regard, the cellsize (Tb.Sp) of samples #1521-55 and #5121-59, on average, is larger(less than about 4 mm) than the cell size of typical human cancellousbone (less than about 2 mm), but the volume fractions (BV/TV) are withinsimilar ranges.

Some of the other morphology data is outside the ranges of the vertebralbone data provided. For example, trabecular number (Tb.N) for thesamples is generally lower than that of the human cancellous bone dataprovided; trabecular thickness (Tb.Th) is generally higher than that ofthe human cancellous bone data provided; trabecular spacing or cell size(Tb.Sp), mentioned above, is generally higher than that of the humancancellous bone data provided; and intercept length (MIL1,2,3) isgenerally higher than that of the human cancellous bone data provided.However, there is some correlation between these values for theartificial cancellous bone samples and human cancellous bone data. Forexample, the ratio of trabecular thickness (Tb.Th) to trabecular spacingor cell size (Tb.Sp) for sample #1521-59 is within the range for humanvertebral cancellous bone Data 2 and human cancellous bone Data 3. Thiscorrelation is likely a result of the larger trabeculae cell size forartificial cancellous bone compared to that of human cancellous bone, asbest seen in FIGS. 10A-10C, and the increased thickness of theartificial cancellous bone trabeculae compared to that of humancancellous bone, as a result of the resin coating. Moreover, the degreeof anisotropy for the #1521-55 and #1521-59 samples are within the rangeof anisotropy for human vertebral cancellous bone Data 2. The degree ofanisotropy is directed to the shape of the pores, with a value of 1.0being perfectly round. The correlation between the artificial cancellousbone samples and human cancellous bone data indicates that bothartificial and human cancellous bone have generally rounded pores.

EXAMPLE 2 Static Compressive Properties

Static compressive properties for samples of rigid open cell foam formedin accordance with embodiments of the present disclosure and humancancellous bone are shown below in TABLE 2.

The method steps for preparing samples #1521-55 and #1521-59 aredescribed above in EXAMPLE 1. Samples RD1, RD2, RD3, and RD4 wereprepared using similar processes, including the following method steps:(1) preparing an epoxy mixture (specific mixture described below); (2)recording the weight of the dry formable open cell foam having 14 ppi(pores per inch), manufactured by the EN MURRAY COMPANY, and sold asproduct PTA 14 ppi Natural having a density of 0.023 g/cm³; (3) soakingthe formable open cell foam sample with the epoxy mixture; (4) feedingthe saturated foam sample through two aluminum rollers that act like asqueegee to remove excess epoxy; (5) recording the weight of wetformable open-cell foam sample; (6) if the weight of the wet formableopen-cell foam was too high or low, then adjusting the distance betweenthe rollers and repeating the squeegee process until the sample of foamis within a specified weight range, based on dry density; and (7)hanging the open cell foam samples in a ventilated area until the wetfoam has cured.

Sample RD1 uses a resin system sold under the brand names RENLAM®4017resin and Ren® 1510 hardener as a heat resistant laminating system,manufactured by HUNTSMAN™. The application of RD1 resin to the dryformable open cell foam results in a hardened open cell foam having anapparent density of about 0.11 g/cm³. Sample RD2 uses a resin systemsold under the brand names RENLAM®4017 resin and Ren®1510 hardener as aheat resistant laminating system, manufactured by HUNTSMAN™, combinedwith 3032 milled e-glass fiber, manufactured by FIBERTEC™, in a 1:1ratio with the resin portion of the system. The application of RD2 resinto the dry formable open cell foam results in a hardened open cell foamhaving an apparent density of about 0.18 g/cm³. Samples RD3 and RD4 useda resin system sold as EC-433 High Temp Epoxy Casting System AluminumFilled and EC-433-2 Hardener, manufactured by ADTECH PLASTIC SYSTEMS.The application of RD3 resin to the dry formable open cell foam resultsin a hardened open cell foam having an apparent density of about 0.31g/cm³, and the application of RD4 resin to the dry formable open cellfoam results in a hardened open cell foam having an apparent density ofabout 0.15 g/cm³.

As described above, the compressive properties for human cancellous bonewere compiled from mean data for 1133 compressive test samples performedon either cylindrical cores or cube specimens of human cancellous bonetaken from vertebrae, femurs, and tibias from cadaver bone aged 20-100years, using a method with uniaxial compressive loading at variousstrain rates. Large variations in the mechanical properties of humancancellous bone, as shown in TABLE 2 below, may be a result ofdifferences in subject age, degeneration, bone density, and source ofbone (e.g., vertebral, tibial, etc.).

The compression properties for the rigid open-cell foam samples RD1 andRD2 listed in TABLE 2 below were determined by compressive testsconducted using ASTM D1621 “Compressive Properties of Rigid CellularPlastics” as a guide. In that regard, square blocks 76.2 mm by 76.2 mmby 40 mm were loaded to failure under displacement control (4 mm/min)using an INSTRON® 4204 load frame with a 50 kN load cell. Specimens wereconditioned at 77° F. for 4 hours prior to testing. The dimensions ofeach specimen were measured with digital calipers and averaged overthree measurements. A preload of 0.02 MPa was applied to each specimento initiate contact with the surface and zero the displacementmeasurement. Load versus displacement data was collected using anINSTRON® chart recorder.

The compression properties for the rigid open-cell foam #1521-55,#1521-59, RD3, and RD4 samples listed in TABLE 2 below were determinedby compressive tests conducted using a conventional mechanical testprotocol used for cellular materials and cancellous bone as described inthe following references: Zhu, M., Keller, T. S., Spengler, D. M.,“Effects of specimen load-bearing and free surface layers on thecompressive mechanical properties of cellular materials,” J. Biomech.27:57-66, 1994; and Keaveny, T. M., Borchers, R. E., Gibson, L. J.,Hayes, W. C., “Trabecular bone modulus and strength can depend onspecimen geometry,” J. Biomech. 26:991-1000, 1993, the disclosures ofwhich are hereby incorporated by reference. In that regard, cylindricalcores 15 mm diameter by 30 mm height were loaded to failure underdisplacement control (1% strain/second) using an INSTRON® 4204 loadframe with a 50 kN load cell. The dimensions of each specimen weremeasured with digital calipers and averaged over three measurements.Specimens were potted with end caps of 2 mm polyester resin manufacturedby DYNATRON®/BONDO CORP. to reinforce the load-bearing surfaces.Specimens were conditioned at 77° F. for 4 hours prior to testing. Apreload of 0.02 MPa was applied to each specimen to initiate contactwith the surface and zero the displacement measurement. Load versusdisplacement data was collected using an INSTRON® chart recorder.

The inventors have found that the two compression test methods describedabove achieve similar results for comparative analysis. For example,compression testing for samples #1521-55 and #1521-59 using ASTM D1621“Compressive Properties of Rigid Cellular Plastics” as a guide resultedin the following compressive values: #1521-55, apparent density 0.09g/cm³; apparent modulus 6.2 MPa, and ultimate stress 0.11 MPa; #1521-59,apparent density 0.12 g/cm³; apparent modulus 18.6 MPa, and ultimatestress 0.28 MPa. These results can be compared with the results in TABLE2 below achieved by using a conventional mechanical test protocol usedfor cellular materials and cancellous bone, as described in thefollowing references: Zhu, M., Keller, T. S., Spengler, D. M., “Effectsof specimen load-bearing and free surface layers on the compressivemechanical properties of cellular materials,” J. Biomech. 27:57-66,1994; and Keaveny, T. M., Borchers, R. E., Gibson, L. J., Hayes, W. C.,“Trabecular bone modulus and strength can depend on specimen geometry,”J. Biomech. 26:991-1000, 1993.

TABLE 2 COMPRESSIVE PROPERTIES Human Cancellous Rigid Open-Cell FoamSamples Bone 1521- 1521- Min Max 55 59 RD1 RD2 RD3 RD4 Apparent 0.090.64 0.08 0.10 0.11 0.18 0.31 0.15 Density (g/cm³) Apparent 26 673 5.212.4 28.9 72.0 202 33.8 Modulus (MPa) Ultimate 0.32 46 0.09 0.24 0.380.59 2.6 0.41 Stress (MPa)

The compression properties for the rigid open-cell foam samples listedin TABLE 2 above can be compared to the mean minimum and maximumcompression properties for human cancellous bone. In that regard, theapparent density for all of the rigid open-cell foam samples fall withinthe minimum and maximum apparent density ranges for human cancellousbone, 0.09 to 0.64 g/cm³. However, only the apparent modulus for theRD1, RD2, RD3, and RD4 rigid open-cell foam samples fall within theminimum and maximum apparent modulus ranges for human cancellous bone,26 to 673 MPa. Further, only the ultimate stress for RD1, RD2, RD3, andRD4 rigid open-cell foam samples fall within the minimum and maximumultimate stress properties for human cancellous bone, 0.32 to 46 MPa.

EXAMPLE 3 Modulus Degradation During Fatigue Loading

Results from the fatigue tests performed on artificial cancellous bonesample #1521-59 are plotted in FIG. 11, as compared to a curve-fit ofdegradation data for human cancellous bone. This data shows thatartificial cancellous bone sample #1521-59, when subjected to dynamicstrength tests, has similar general behavior (non-linear modulusdegradation) for normalized apparent modulus of elasticity per lifefraction when compared to human cancellous bone data, as published inPattin, C. A., Caler, W. E. Carter, D. R., “Cyclic Mechanical PropertyDegradation During Fatigue Loading of Cortical Bone,” J. Biomechanics29:69-79, 1996, the disclosure of which is hereby incorporated byreference.

EXAMPLE 4 S-N Curve Behavior

Results from the effective strain tests performed on artificialcancellous bone samples #1521-59 and #1521-55 are plotted in FIG. 12 andcompared to effective strain tests performed on bovine trabecular bone,human vertebral bone, and closed cell foam, showing similar S-N curvebehavior. Samples #1521-59 and #1521-55 fall within the same effectivestrain range as human cancellous bone. Closed cell foam shows adifferent effective strain range than human cancellous bone. Themammalian data was published in Bowman, S., Guo, X., Cheng, D., Keaveny,T., Gibson, L., Hayes, W., McMahon, T., “Creep Contributes To theFatigue Behavior of Bovine Trabecular Bone,” J. Biomech. Eng.120:647-653, 1998; Haddock, S., Oscar, Y., Praveen, M., Rosenberg, W.,Keaveny, T., “Similarity in the Fatigue Behavior of Trabecular BoneAcross Site and Species,” J. Biomech. 37:181-187, 2004; Palissery, V.,Taylor, M., Browne, M., “Fatigue Characterization of a Polymer Foam toUse as a Cancellous Bone Analog Material in the Assessment ofOrthopaedic Devices,” J. Mat. Sci. & Mat. Med 15:61-67, 2004; andRapillard, L., Charlebois, M., Zysset, P. K., “Compressive FatigueBehavior of Human Vertebral Trabecular Bone,” J. Biomechanics[e-publication], 2005, the disclosures of which are hereby incorporatedby reference.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the disclosure.

1. An artificial bone, comprising: (a) an outer wall defining an innercavity; and (b) an inner core disposed within at least a portion of theinner cavity, wherein the inner core includes a porous material havingstiffness within a range of stiffness properties for mammaliancancellous bone and strength within a range of strength properties formammalian cancellous bone, wherein the inner core includes a barrierlayer to separate the outer wall from the porous material.
 2. Theartificial bone of claim 1, wherein the inner core has stiffness,measured as an apparent modulus of elasticity, within a range selectedfrom the ranges consisting of at least about 26 MPa, about 26 MPa toabout 673 MPa, about 55 MPa to about 535 MPa, about 26 MPa to about 200MPa, and about 26 MPa to about 100 MPa.
 3. The artificial bone of claim1, wherein the inner core has an apparent density within a rangeselected from the ranges consisting of at least about 0.9 g/cm³, about0.09 to about 0.64 g/cm³, about 0.09 to about 0.49 g/cm³, about 0.12 toabout 0.49 g/cm³, and about 0.15 to about 0.49 g/cm³.
 4. The artificialbone of claim 1, wherein the inner core has strength measured byultimate stress within a range selected from the ranges consisting of atleast about 0.32 MPa, about 0.32 MPa to about 46 MPa, about 0.34 MPa toabout 12.08 MPa, about 0.34 MPa to about 1.0 MPa, and about 0.34 MPa toabout 0.60 MPa.
 5. The artificial bone of claim 1, wherein the innercore comprises rigid open cell foam.
 6. The artificial bone of claim 5,wherein the rigid open cell foam is an open cell foam saturated with ahardening agent, selected from the group consisting of resin, ceramic,metal, and any combination thereof.
 7. The artificial bone of claim 1,wherein the inner core has about 10 to about 40 pores per inch.
 8. Theartificial bone of claim 1, wherein the inner core has about 10 to about20 pores per inch.
 9. The artificial bone of claim 1, wherein thebarrier layer is a plastic film.
 10. The artificial bone of claim 1,wherein the outer wall is substantially continuous.
 11. An artificialbone, comprising: (a) an outer wall defining an inner cavity; and (b) aninner core disposed within at least a portion of the inner cavity,wherein the inner core comprises a porous material having an apparentmodulus of elasticity of at least about 26 MPa and ultimate stress of atleast about 0.32 MPa.
 12. The artificial bone of claim 11, wherein theinner core includes a barrier layer to separate the outer wall from theporous material.
 13. The artificial bone of claim 11, wherein the outerwall is substantially continuous.
 14. The artificial bone of claim 11,wherein the inner core comprises a porous material having an apparentdensity of at least about 0.09 g/cm³.
 15. A method of making anartificial bone, comprising: (a) obtaining an inner core, wherein theinner core includes a porous material having stiffness within a range ofstiffness properties for mammalian cancellous bone and strength within arange of strength properties for mammalian cancellous bone; (b)substantially covering the inner core with a barrier layer; and (c)molding a substantially continuous outer wall around the inner core. 16.The method of claim 15, wherein the outer wall includes material isselected from the group consisting of polyurethane, and fiberglassfilled epoxy resin.
 17. The method of claim 15, wherein the barrierlayer is a plastic film.
 18. The method of claim 15, wherein the innercore comprises rigid open cell foam.
 19. A method of making anartificial bone, comprising: (a) obtaining first and second outer wallportions, wherein each of the first and second outer wall portionsdefine a portion of an inner cavity such that, when the first and secondouter wall portions are assembled into an outer wall, the outer walldefines an inner cavity; (b) obtaining a hardened inner core configuredto fit within at least a portion of the inner cavity; (c) inserting theinner core within at least a portion of the inner cavity of the firstouter wall portion; and (d) attaching the second outer wall portion tothe first outer wall portion to form the outer wall having the innercore disposed within the at least a portion of the inner cavity.
 20. Themethod of claim 19, wherein the inner core comprises rigid open cellfoam.
 21. A method of making an artificial bone, comprising: (a)obtaining an outer wall portion, wherein the outer wall portion definesat least a portion of an inner cavity; (b) obtaining an inner coreconfigured to fit within at least a portion of the inner cavity, whereinthe inner core comprises a formable porous material; (c) inserting theinner core within at least a portion of the inner cavity of the firstouter wall portion; and (e) hardening the inner core after inserting theinner core within the at least a portion of the inner cavity of theouter wall portion.
 22. The method of claim 21, wherein the inner corecomprises rigid open cell foam.